Fender Princeton Reverb clone, clipping problem when connecting reverb tank.

[varaso]

Hello, It's my first amp built after too many stompboxes and I built myself a very nice Princeton Reverb clone, kind of an upgraded version that sounds amazing clean, but I have an issue when the thing gets louder. Whenever the amp starts clipping with the volume over halfway, and hitting the strings hard, a strange noise occurs, a scratchy kind of sound. It only happens when it's clipping and it disappears completely only when I remove the Input of the reverb tank. Other than that, the reverb works as it should even when that scratchy thing happens, and when the tank is disconnected the clipping sounds normal, and when cranked you get a nice rich in harmonic content tube overdrive sound. I tried another 12AT7 tube, another reverb tank and it does the same thing with the tank inside or outside the cabinet. No luck.. All the voltages measure ok with a GZ34 rectifier tube, a Vibrolux speced PT, a Deluxe Reverb Hammond 1760H OT and a Hammond 1750A Reverb OT. I hope someone with his genius help me make this amp as awesome as it's meant to be! Thanks in advance!

P.S. I can post voltages later (in about 12 hours because it's too late in Greece) and even record the problem to give you a better understanding of the issue.

[pdf64]

Scratchy brings to mind oscillation.
A sound clip, photos and voltage chart would be helpful.
With such issues, layout and lead dress are significant.
Grid stoppers can help.

[Adrien]

I had the exact same problem with mine, this really horrible harsh distortion at higher volumes.  How I fixed it was by simply using a 12AU7 in place of the AT.  There's still plenty of reverb and no more nastiness.

This is apparently caused by the reverb driver clipping, read this thread for more info and other possible solutions:  https://el34world.com/Forum/index.php?topic=23084.0

[pdf64]

I think that the clipping caused by the reverb driver is a different issueto that noted in the OP by varaso.
With a stock BF amp, reverb clipping of the dry signal path occurs at signal levels above those at which the power tubes enter grid clipping (ie it is masked by the power tube grid cliiping), hence it is not a characteristic which is readily audible, and generally only becomes apparent when a master volume, especially a type 4, is used (as noted on Skydragon's thread, and with many later SF reverb amp models).

However, the fixes for reverb driver grid clipping (eg grid stoppers, mods to lower the gain / HF extension somewhere in the reverb loop) will likely also increase the amp's margin of stability, and so may help varaso's issue.

[varaso]

Thanks for your answers! I'll open the amp up in a bit and will try some things like removing the bypass cathode cap and some other things suggested in the link above.  The issue should be the same with the deluxe's because it stopped when he unplugged the reverb tank input.

[varaso]

I disconnected the cathode cap and the noise went away, but the reverb fainted because of the gain drop. Also my voltages are shown below

V1

1: 169
3 : 1,27
6: 167
8: 1,34

V2

1,6 : 420
3,8: 7,76

V3

1: 161
3: 1,4
6: 164
8:1,52

V4

6: 250
8 : 72

Filter cap voltages:
437
426
330
260

PT Output voltage 352  -0- 352 kind of higher than it should be (325) , it was a custom order from a local transformer store.

[varaso]

https://drive.google.com/open?id=18kJDCaS0z3A-8tY2dNcA8ijSMnhbm5Z9

here is the sound clip, you can hear the noise starting at 0:26. At 0:50 I've the reverb tank input disconnected and the problem goes away. Finally at 1:05 i reconnect the reverb tank.

[Adrien]

Thanks for the clip.  That's exactly what mine sounded like, I guess I didn't describe it very well in my post above.  On mine the noise also went away when I disconnected the reverb tank, and my voltages are similar to yours.  What was the volume set to on the amp?  With mine it would happen above 4 or 5 on the volume now that I think about it.  I guess my initial description wasn't very accurate at all...

I don't know enough to offer any concrete explanations for why this problem might be occurring (I'm sure others on here can), but again, I'd suggest trying a 12AU7 in v2 instead of the 12AT7 if you've got one.  It's certainly a bandaid fix and doesn't address the root cause, but that's what worked for me.  It's still going strong 3 years later... 

[pdf64]

Listened to the clip, I think the issue is positive feedback causing parasitic oscillation, ie the unpleasant distortion is little blips of oscillation riding on the main signal waveform; it's not free running oscillation.
The exact schematic, layout, and build photos are needed to advise what to do next sort it out.
A simple step that will likely resolve it is to add grid stoppers, eg 10k, to each grid at the socket terminal.
Ask if you're unclear about what that means.
The original BF layout and lead dress was likely fine tuned over numerous iterations to achieve stable operation without need for grid stoppers (each component increased cost!).

[varaso]

Well I put a 560p 3kV cap across the anode and the cathode of the reverb driver tube and it did the trick. Though I'd like to know why the amp did't play perfect without this mod. I used a tad pdf file, just becasue it was easily accessible and with fine resolution, other than that I'm very disappointed with their products! 

http://www.tubeampdoctor.com/images/artikel/Full_documentation_BF-14-REV_2.pdf

[pdf64]

What TAD stuff didn't you like?

Grid stoppers on the reverb driver V2 would likely have sorted it too, and they would also help with the reverb clipping issue, so may act to improve the overdriven tone.

[varaso]

Before fixing that problem I thought that the TAD 12AT7 was bad, because it made some weird noises other than the weird noise I fixed and the TAD rectifier tube rattles very loudly and resonates with the cab. After all, I put the TAD 12AT7 back in and find out it's a very very nice tube and the reverb is perfect! So they restored some of their reputation in my mind, though the rectifier still resonates but I can live with that since I don't record often. I also had a pair of el84 tube in a Blues Jr that I expected to last longer, though I used the amp intensely. So after all I think they're ok, perhaps they'll have another chance in the future hehe..

Perhaps I'll try to use grid stoppers just to experience the results, since it's such an easy thing to unsolder the cap and put a grid stopper..

[varaso]

Well, recently I found out that when I put the volume on full and the Treble on full (all the other pots don't interact) I get a weird ticking sound that goes away when the treble is backed down at 3 o'clock or the Volume at about 85-90% OR if i just disconnect the Input of the Reverb tank !! If I just disconnect the Output of the reverb tank the ticking noise is just a little less intense.

 Any thoughts? The only mod I did to fix the clipping issue I had in the beggining with is putting a 560p cap across the anpde and the cathode of the 12AT7 Reverb Tube .

[shooter]

disconnect the Input of the Reverb tank

what happens when you leave it connected but have the reverb pot on 0

[varaso]

The same thing.. the reverb knob doesn't affect the weird noise.. That's a little clip I recorded :

https://drive.google.com/open?id=1aXyZMzxbBvkvMxNWj3nA3uD6MXgfiLjF

until 0.25 I tweek only the Treble and Volume controls.. then I disconnect the reverb out and the noise becomes less intense and at 0.28 I remove the reverb in.

[shooter]

https://el34world.com/Hoffman/files/Hoffman_PrincetonReverb_NoTremolo.pdf

without a schematic this is the one I'm working from.  I'd make it a no-reverb amp temporarily, I think you probably have 2 issues since you said;

I disconnect the reverb out and the noise becomes less intense

time to bust out the scope

[pdf64]

That sounds like motorboating, ie free running oscillation due to a positive feedback loop via the HT.
Maybe due to a bad HT decoupling cap, too many circuits sharing the same 0V return etc. Of course the can cap restricts the ability to implement normal good practice, eg Merlin's multiple star 0V arrangement.
Perhaps try bridging each can cap section in turn with a suitable known good cap.
Any gain boosting mods?

[varaso]

I may have built an amp but I'm still learning electronics, kind of a noob :P

so... What do you mean by HT?

I didn't use a can cap, I used regular new F&Ts 3x22uF and 1x 47uF filtering capacitors. Can I somehow identify the bad one?

I haven't done any boosting mods, but I intent to buy a 4KΩ 50W OT to be able to run in with 6L6s or 6V6 by mismatching the impedance. And I'm thinking about having a switch to choose between the standard or lower value dropping resistors (like Deluxe's or Twin's), so I can have more headroom for big gigs.

[shooter]

HT is Hi tension, big volts, typically the PS (powersupply) rail, and tube plates.
motor-boating is a slow moving oscillation, poor AC filtering, and lots of other things.

If you have an extra filter cap 450vdc or more, put it in parallel with the last cap, in the schematic I ref'd it would be tap D.  That feeds V1, the most sensitive tube.

other MB "fixes" would be, parallel a really big (>100uF) cap with the cathode cap for V1A.

without a scope, we're just throwing stuff at the wall n watching what sticks.

one way to know, if the reverb circuit isn't connected, it can't be your problem.
a quikie test would be to remove the bypass cap on V1A, that'll knock down gain and kinda sorta eliminate clipping (too much gain)(ya, I know I like gain:) as a cause, or not.

 

[varaso]

So, I'm confused..    just took the chasis out and before I plug it in I aligator clipped a 22uF cap in parallel with the last filtering cap, the one that feeds the preamp. I fired it up, cranked the volume and the treble and with the reverb tank connected there was no weird noise. Then I removed the 22uF I had aligator clipped and fired it up again and still the noise wasn't there.. How could the noise go away ? That's confusing.. Well, I'll post back when the issue reappears!

[shooter]

just took the chasis out and before I plug it in

you made a change, a mechanical change, AND symptoms changed, that would point to something loose, a wire moved enough not to couple.... ..... .....

[varaso]

Yeah found out this morning the Reverb cable broke.. I fixed it and alright!! the problem is back ! When I get the chance I'll take the chasis out again and try the same thing with the cap.. Tomorrow I guess!

[Willabe]

Which verb cable?

[varaso]

The inner lug of the reverb output RCA broke because of a hit and it did't made contact. But the shield was still grounded. When I completely removed the reverb output cable the noise reappeared. (The input cable was connected all the time).

[shooter]

I put the volume on full and the Treble on full

removed the reverb output cable the noise reappeared.

same noise

what happens at 9 n 9

[varaso]

When I put it on 9 n 9 I get no noise.. If the volume is cranked and the treble bellow 3 o'clock it's fine.. if the treble is cranked and the volume is at 90% the it's also fine..

[shooter]

then just rock it, make a note to self "9 is the new 10" walla!

otherwise get a scope study many hours on gain stage tweaking, HF control, signal coupling, etc, etc.

[varaso]

Hehe yeah I don't think I'll ever need to crank the amp but it bugs me you know.. to know that you just can't do it.. and it also means there's a fault somewhere, maybe mine, maybe a component's..

I found out that the issue remains even after I remove the reverb tank completely and I can't play the amp cranked at my home it's too loud even for just a bit of testing..

also when the vol pot is 0 and I increase the treble pot at 80% it makes an instant pop noise .. the same thing happens when the treble is full and I increase the volume at about 5%.

I put a cap in parallel to the filter cap that feeds the preamp and it made no difference at all. I checked the filter caps and measured them, and they seemed good..

Does ground loops have to do anything with that? because everything is grounded to the chasis in different places using solder lugs that are bolted to the chasis or to the back of the pots.. and the chasis is just inox, nothing fancy..

I guess I'll just let it be for now.. I'll play it the weekend at our rehearsal place and if the issue is worse than I imagined perhaps I'll post back my observations.

The think is I intend to put some 6L6s/KT66 in the near future with a OT swap and I hope the issue won't get worse..